George already suspects that Gatsby is responsible for Myrtle's death when Tom tells George where to find Gatsby.
At this point, Tom knows that it was not Gatsby who was driving when Myrtle was hit. Daisy was driving the car - Gatsby's car - and it was Daisy that killed Myrtle.
Daisy was driving the car that Myrtle ran in front of because she probably believed that Tom was in it.
Tom is fully aware of this. He has a chance to tell George not to go looking for Gatsby, to tell him that Gatsby did not kill Myrtle, or to lie or simply be silent about Gatsby's whereabouts.
Tom tells Nick that George is imbalanced when he comes looking for Gatsby. Tom is afraid that George will kill him or kill Daisy. He tells Nick that this reasonable fear for his own life led him to give George the information he came looking for.
At this time, George also believes that Gatsby was Myrtle's lover. Tom does not correct this misconception either.
Tom is indirectly responsible for Gatsby's death because he uses Wilson's hatred and jealousy against Gatsby in making Wilson believe that Myrtle was Gatsby's mistress.